1. Field of the Invention
The present invention relates to an image forming apparatus such as a copier, a printer, and a facsimile machine, and more particularly, to an electrophotographic image forming apparatus.
2. Discussion of the Background
Various types of transfer materials, such as copy sheets, are used in an image forming apparatus such as a copier, a printer, a facsimile machine, and the like. Depending on the purpose of image formation, a transfer material that has a rough surface, that is, having surface asperities, is sometimes preferred.
However, when such a rough-surfaced transfer material (hereinafter “rough material”) is used for printing, there is a possibility that an image formed thereon might be disturbed during the transfer process, which is a process of transferring a toner image onto the transfer material.
In particular, where the surface asperities are relatively significant, in a concavity, a gap is present between the transfer material and the toner image that is formed on the toner image carrier that carries the toner image, such as a photoreceptor and an intermediate transfer belt, destabilizing a transfer electric field and resulting in image failure, such as a white void in which toner is partly absent, inconsistencies in lightness, and image density unevenness.
FIGS. 10A and 10B are schematic views illustrating an area surrounding a concavity in the surface of the rough material when a filled-in image or solid image patch is transferred from the image carrier to the transfer material using a known image forming apparatus. FIG. 10A shows a state before the transfer process and FIG. 10B shows a state after the transfer process.
Referring to FIGS. 10A and 10B, it can be seen that when rough material, such as Japanese paper, is used for printing, the transfer electric field cannot be formed sufficiently because of the gap created by the concavity.
In other words, because an electric charge applied from a back surface of the transfer material by the transfer bias is too far from the toner image due to the gap, the toner image carried by the image carrier cannot be sufficiently attracted by electrostatic force to the front surface of the transfer material. Therefore, substandard images, such as images whose image density is uneven, are the result.
Several approaches described below have been proposed to prevent such image failure.
In one known image forming apparatus, an image carrier such as an intermediate transfer belt is vibrated by ultrasound to weaken adhesion between the toner and the image carrier so that the image can be transferred to the transfer material even if the electric field is unstable in the gap portion.
However, in such an image forming apparatus including a vibration member, vibration noise is generated, which can annoy users. Additionally, the vibration tends to shorten the working life of other members such as the image carrier.
In another known image forming apparatus, to print high quality multicolor images, the image carrier such as an intermediate transfer belt includes an elastic layer, and its surface that carries toner is designed to have a surface micro hardness within a predetermined range to follow the asperities in the surface of the recording medium, thus reducing the gap.
However, in this known image forming apparatus, the cost of forming the elastic layer on the image carrier is relatively high. Further, this configuration cannot accommodate tiny gaps.
In another known method, the image forming apparatus is a direct transfer type. The image forming apparatus includes an information acquisition mechanism that acquires information related to a surface structure of the transfer material, and a control mechanism that varies the degree of toner adhesion depending on the degree of surface roughness of the transfer material. Then, when a sheet reading mechanism in the information acquisition mechanism detects that the sheet has a rough surface, the control mechanism increases a transfer bias that is applied to a transfer nip during the transfer process by the control mechanism so as to increase the amount of the toner adhering to the sheet.
However, in this known image forming apparatus, although the sheet reading mechanism judges whether the surface of the sheet is rough or smooth, the judgment is made in accordance with the entire surface of transfer material, and adhesion is adjusted by varying the transfer bias.
Therefore, because this mechanism does not adjust the toner adhesion amount in accordance with localized concavities of the surface of the sheet, the overall color reproducibility has a problem, and the solid shaded areas and halftone are not balanced.
Additionally, because the toner adhesion amount is increased for the entire transfer material, the developer is consumed in excess, which is inefficient.